There are many situations that temperature has to be monitored in manufacturing and everyday life, for example, in the storage and transportation of food, medicine, chemical medicine, temperature sensitive commodity, environment temperature are monitored thus to insure the environment temperature conform to the requirements. There are plurality of thermometers capable of indicating, recording environment temperature, and to generate a sound, light or other alarm signals, or to output corresponding temperature record by printing graphs, when temperature reach a predefined lower threshold and/or a predefined upper threshold. However, these products often require power supply, it is complex and voluminous, and the most important issue is cost inefficiency.
Currently, there is an over temperature alarm indication product without power supply, that is colour-variable temperature indicating label. This product does not require any external power supply, may stick on the object to be monitored directly to change colour according to the changes of environment temperature. Therefore, the observer may estimate whether the surface temperature of the object has exceeded the upper threshold according to the colour of the label. The disadvantages of this product are: low in indication precision, normally with ±2° C. error. The temperature coverage is normally between 50° C.-120° C.; in addition, the temperature indication step is often 5° C., therefore it is difficult to indicate a random temperature through the change of its colour; further, the cost of the product is high, therefore, it is difficult to popularize.